This invention relates to the preparation of diaryl carbonates by oxidative carbonylation. More particularly, it relates to the improvement of diaryl carbonate yield in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply "carbonylation" for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is a Group VIIIB metal, i.e., a metal selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof.
A further development in the carbonylation reaction, including the use of compounds of other metals such as lead or cerium as cocatalysts, is disclosed in various patents including U.S. Pat. No. 5,498,789. Also required according to that patent is the use of quaternary ammonium or phosphonium halides, as illustrated by tetra-n-butylammonium bromide, as part of the catalyst package.
The commercial viability of the carbonylation reaction would be greatly increased if a less expensive compound could be substituted for the quaternary ammonium or phosphonium halide. It has been discovered, however, that substitution of such compounds as sodium bromide normally results in the isolation of the desired diaryl carbonate in low or insignificant yield.
In U.S. Pat. Nos. 5,543,547 and 5,726,340, the use of carbonylation catalyst systems including palladium or an analogous metal, various cocatalytic metals which may include cerium, lead or cobalt, and an alkali metal or quaternary ammonium bromide is disclosed. Also present may be materials characterized as inert solvents. These may be aliphatic or alicyclic hydrocarbons such as hexane, heptane or cyclohexane; chlorinated aliphatic hydrocarbons such as methylene chloride or chloroform; aromatic hydrocarbons such as toluene or xylene; chlorinated aromatic hydrocarbons such as chlorobenzene; ethers such as diethyl ether, diphenyl ether, tetrahydrofuran or dioxane; esters such as ethyl acetate or methyl formate; nitroaromatic compounds such as nitrobenzene; or acetonitrile. There is no suggestion, however, that yields of diaryl carbonate are in any way improved by the use of any of these "solvents" in a halide-containing catalyst package.
U.S. Pat. No. 5,380,907 discloses the use of a nitrile in combination with palladium and a manganese or copper cocatalyst. The result is an increase in yield, but yields are still too low to permit contemplation of commercial use for the disclosed catalyst systems.
It is of interest, therefore, to develop catalyst systems which include an inexpensive halide compound and which can efficiently produce diaryl carbonates.